Selection of patients with extracranial oligometastatic disease who may benefit from metastasis-directed stereotactic body radiation therapy (SBRT) is a challenge. This multi-institutional retrospective analysis sought to identify clinical and demographic factors associated with improved OS in patients treated with SBRT for oligometastatic disease.

We conducted a multi-institutional, retrospective chart review of patients treated with SBRT to ≤5 extracranial metastatic lesions with curative intent. Patients with all primary tumor types and histologies were included. Clinical and demographic factors associated with improved OS were examined using univariate and multivariate Cox proportional hazards models. We used the results of this model to create a risk stratification schema estimating survival across distinct categories of number of risk factors.

A total of 371 patients treated from 2006-2015 met entry criteria. Median follow-up was 1.5 years (range, 0.5 -- 9). Median OS was 1.7 years (range, 0.5 -- 9.1), and 5-year OS was 33.8% (95% CI, 26.0% - 41.7%). On univariate analysis, Karnofsky Performance Score (KPS) > 80 (HR 0.42, p<0.001), metachronous metastasis (HR 0.57, p=0.006), prior local therapy for metastasis in the same organ (HR 0.49, p=0.02), time from primary diagnosis to SBRT-treated metastasis (HR 0.996, p=0.04), and breast, colorectal, kidney, and prostate primary compared to lung primary (p<0.05) were associated with improved OS. Solitary metastasis also showed a trend for improved OS (HR 0.69, p=0.08). On multivariate analysis, KPS > 80 (HR 0.44, p=0.001), solitary metastasis (HR 0.53, p=0.02), prior local therapy (HR 0.45, p=0.03), and primary tumor organ (p<0.01) were associated with improved OS. Patients with a total of 0-1, 2, or &ge;3 favorable variables from the multivariate model had 2-year OS of 42.6% (95% CI, 26.3% - 57.9%), 60.8% (49.7% - 70.3%), and 83.3% (72.5% - 90.2%), respectively.

Using a large multi-institutional cohort of patients who received SBRT for extracranial oligometastatic disease, we found that KPS>80, solitary metastasis, prior local therapy, and primary tumor organ can be used to stratify patients according to probability of survival. Future validation studies are needed.